This invention relates to the field of electromagnetic energy aperture arrays that are useable in the communication of optic and radio frequency signals, particularly to arrays of reduced sidelobe energy content.
Large diameter radio frequency antennas or optical radiators offer desirable signal characteristics for use in weapons systems, outer space astrophysical observation equipment and in other electromagnetic radiation systems. The physical size and weight attending such large diameter apertures is often, however, prohibitive in view of the physical constraints imposed by the operating environment of many using systems. The transportation of a 20 foot diametered microwave antenna into outer space or the battlefield use of a laser target illuminator which includes a 10 foot diametered optical reflector can be understood to be physically attainable but attended by practical complexities and undesirable complications. Severe aperture size limitations also frequently arise in systems which are required to perform high rate angle tracking, i.e. in system such as anti-aircraft fire control radars.
A possible resolution to these aperture difficulties lies in the use of an array of physically smaller apertures which cooperate to provide an electromagnetic signal of similar or even improved characteristics over that which attends the single large aperture arrangement.
In the case of microwave antennas as might be employed in a high powered microwave (HPM) weapon or also in the case of such apparatus as a microwave target illuminating system, it is desirable to maximize the energy delivered to a central target from a high gain antenna. Such antennas often include a parabolic dish that is disposed in a cassegrain configuration--an antenna arrangement wherein the energy source is mounted at the focal point of a reflector and aimed toward the reflector in order to reflect a maximum amount of energy to the target and realize minimal energy incidence on angularly adjacent non-target areas.
In the weapons environment, the presence of undesirable sidelobe energies can easily lead to fratricide, the subjection of friendly forces to unintended damage through sidelobe illumination of objects that are located in small angular separation from an intented target. The use of small subaperture antennas as described herein-apertures that are driven from phase-locked energy sources and have known main lobe to sidelobe ratios can provide HPM radiation patterns having a minimal probability of damage to unintended objects. In the present invention it is assumed that the desired subaperture array is to be inscribed within the physical confines of the replaced larger circular aperture with the number of subapertures and their physical disposition being selected to optimize the attained illumination energy pattern.
In describing the performance of the herein disclosed multiple subaperture arrays it is convenient to compare energy distribution patterns with the far-field Airy pattern of an equivalently diametered single aperture array, an array which corresponds to the single antenna dish or a single optical collector. Moreover in the optical environment, the performance of single aperture and multiple subaperture systems can be compared on the basis of far field impulse responses of the single aperture and multiple aperture arrays.
As one means of evaluating the performance of multiple subaperture systems and comparison with a single apertured system according to this arrangement, one can consider the single aperture central lobe width of an imaged point source with respect to the central lobe width of an imaged point source arising from the multiple subaperture configuration being considered. The subaperture performance is considered to be good when the central lobe width of its imaged point source is equal to or less then that of that of the single large aperture arrangement. Good performance is also characterized by secondary irradiance maxima which are less than 0.0175 of the central lobe peak irradiance.
The prior patent art indicates significant attention of inventors to the use of multiple aperture arrays and improvement of their main lobe and sidelobe energy distributions for a variety of electromagnetic systems. In the radio frequency antenna art, for example, the patent of H. Yagi U.S. Pat. No. 1,860,123 reveals the use of a plurality of spatially oriented antenna elements in an electric wave generating apparatus. The patent of E.A. Ohm. U.S. Pat. No. 4,236,161, also teaches the use of antenna apertures which are arranged in plural groups of seven antennas. Some of the Ohm antennas in each group are common to multiple groups.
In the optics art, patent of E.E. Hadley. U.S. Pat. No. 3,502,387, shows a plurality of telescopes having multiple primary mirrors whose fields of view overlap. In the Hadley patent, an overlapping display provides a single image with a larger field of view--an arrangement which is illustrated in FIG. 6 of the Hadley drawings.
Also in the optical art, the patent of J.S. Fender et al, U.S. Pat. No. 4,639,586, shows the use of phased apertures in a laser transmitter apparatus. FIG. 16 in particular shows a seven element phased array of telescopes. The patent of A. Wirth et al, U.S. Pat. No. 4,725,138, is concerned with a wave front sensor in which the wave front is divided into a plurality of subapertures and passed through a filter array 10 which comprises a mask having as many mask cells as subapertures and also having a lenslet lit array 12 which is used to divide the input pupil into subapertures.
In the acoustic art, the patent of O.H. Schuck, U.S. Pat. No 2,837,728, teaches the use of acoustic transducer elements which are placed in a predetermined concentric circle array pattern. An underwater transducer of the sonar type is one possible use of the Schuck apparatus.
None of these prior art patent references, however, teaches the arrangement of subaperture elements as set forth in the instant invention.